If the kittens are nursing well, then I find starting the kittens on solid foods at 6 weeks old is a lot more successful. Up to that age, they really are not interested in eating solid food and you frustrate yourself more worrying about it!
My Blossom was abandonded at 3 weeks when my son found her. I took her to the vet's & he told me to start introducing her to wet food. I thought it was too young & kept her on the bottle a couple of weeks & then started her. I've heard that the mumma cat will show her kittens when she's ready & they are interested.